Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.
The collision energy dependence for Br(2P3/2) atom formation in the reaction of OD + HBr has been investigated from 0.05 to 0.26 eV using a crossed molecular beam experiment. OD radicals were selected as the single rotational state in the upper state of Λ-doubling of using a 1 m electric hexapole field. Br atoms were detected by the vacuum ultraviolet (VUV) laser-induced fluorescence technique. We find that the reaction cross-section decreases, increasing the collision energy. This negative collision energy dependence suggests that there is no barrier on the potential energy surface for the formation pathway considered. Results were compared with those previously reported for the OH + HBr reaction system. We find that the ratio of the reaction cross-section of σ(OD)/σ(OH) shows values larger than one and an increasing tendency when collision energy increases. The collision energy dependence observed is explained in terms of the zero-point energy differences and the rotational periods of OD and OH, which may be related to the time for the proper reorientation of the OH radical prior to the reaction.
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